Blue Milk?

Secrets to milk’s color are all about deflection.

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iStockPhoto.com/Peter-John Freeman

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Milk and snow seem to have the white market cornered. In many oral cultures, if you wanted to give something that special glow, you associated it with milk: If you rode away on a steed or charger, chased a boar, or even spent some time with a swan, it was practically guaranteed to be milk-white. Even Steven Sondheim’s nursery rhyme characters in Into the Woods look for “a cow as white as milk.”

But why does milk look white? Or perhaps a better question, when I switched to skim milk, lo these many years ago now, why did my cereal take on a bluish tinge?

In order to understand milk’s whiteness, we must first understand a little bit about colors and the way light works. Light is really a form of electromagnetic radiation (radio waves and microwaves are, too). This radiation has a wavelength that can be measured, and each color of visible light has a different wavelength.

The color we see is dependent upon the wavelengths that enter our eye. Grass appears green because it reflects green wavelengths of light, while absorbing other wavelengths. Tar is black because it absorbs all visible wavelengths. White is special: When all the wavelengths are reflected, we perceive the color as white.

When light interacts with suspended particles (such as in air or a liquid), energy can be deflected or absorbed. The size and chemical nature of the particle determine the wavelength that is deflected. This explains the blue of the sky. Small particles in the atmosphere scatter the shorter blue and violet wavelengths sufficiently to cause the sky to appear blue.

Try this experiment: Put water in a medium-size clear glass and shine a flashlight through it. The light beam is very difficult to see in the water. (You may be unable to see it at all.) Add a few drops of milk to the water and shine the flashlight through the glass. The light beam is now visible as it passes through the mixture, due to scattering of the light. Keep adding drops of milk and, eventually, the mixture will take on a distinct blue color because the milk particles are scattering the blue light. If you put a piece of paper on the other side of the glass to catch the remaining light beam, it will be orange.

In simplest terms, milk is a suspension of transparent particles in fluid (containing dissolved proteins, enzymes, vitamins and lactose). The largest of the suspended particles are butterfat globules and micelles, which consist of a protein called casein and fat droplets grouped together with calcium in spheres that contain several thousand molecules. Because these particles are numerous and relatively large, they deflect all the visible light that might otherwise pass through milk. And voilà, your milk appears white.

Virtually all the butterfat has been removed in skim milk, and, unless whiteners are added, only the casein particles remain on deflecting duty. Fewer and smaller, they cause less light to be deflected at longer (red) wavelengths, while continuing to scatter the shorter wavelength radiation. And thus, the milk in my cereal bowl appears blue.


Grit Associate Editor Jenn Nemec likes milk of most colors, enjoys epic poetry and can be captivated by a good musical.

Have a Rural question you always wondered about? Email it to QuizGrit@Grit.com , and we may answer it here.